Cook the Chardonnay and the onion together over a low fire until the wine reduces and the onion is very soft and transparent.
Melt 1 tablespoon of butter in another heavy saucepan over medium heat. Add the flour and whisk until blended. Add the stock a little at a time, whisking constantly until blended and thickened. Add the mushrooms, green pepper and garlic. Smother down until the vegetables are soft.
Remove from heat and add 1/2 cup of butter in pieces until melted. Add to the Chardonnay (which should now be like a glaze) mixture. Blend well over very low heat. Season to taste with salt (may not be needed), black pepper, cayenne, and tarragon.Makes 4 Source: